1. Field Of The Invention
The subject invention pertains generally to method and apparatus for automatically optically scanning coded indicia such as bar codes. In particular, the invention relates to method and apparatus for variable speed scanning of optically readable codes such as bar codes which require minimal operator involvement to achieve successful decoding of bar codes being scanned
2. Description Of The Prior Art
In a variety of present day applications including merchandise warehousing and point-of-sale transactions, information pertaining to the merchandise is obtained by reading data encoded indicia such as an optical code printed on or affixed to the merchandise item These optical codes are typically in the form of UPC or other formats of bar codes having a plurality of linear code elements Various equipment and techniques have been developed for manually reading or automatically scanning and decoding bar codes. Additional discussion of the prior art is included in U.S. application Ser. Nos. 913,098 and 913,215 both filed on Sep. 29, 1986 and assigned to the assignee of the present invention.
The versatility of prior art systems for manually reading or automatically scanning and decoding bar codes has typically been measured by such parameters as the range of densities of bar codes which can be read, the range of distance allowed between the scanning device and the bar code to be scanned, the speed at which the bar code can be read and decoded, and the cost effectiveness of the product. Several techniques have been developed which optimize one or more of these parameters. However, optimization of one or more parameters has typically been achieved at the expense of adequate development of the other parameters, resulting in what may best be described as a compromise suited to only a limited range of applications.
Low cost, compact bar code readers such as those incorporating light pens are available where expense is of primary significance Such readers are typically used in relatively low-volume point-of-sale transactions. The light pens usually use light emitting diodes (LED,s) for providing the scanning signal However, such readers have the disadvantage of being relatively slow and require the operator to position the reader at a fixed distance from the bar code to be read. The light pen must be manually moved across the surface of the bar code at a predetermined speed in order for the bar code to be decoded Because the attainment of the proper speed of pen movement across the bar code is a trial and error process, repeated manual scans are oftentimes required in order to successfully decode the bar code.
Non-contact bar code readers offer a significant advantage over the light pen, especially when an automatic scanning technique is employed For discussions of examples of automatic scanning devices, see, for example, U.S. application Ser. No. 913,215 referred to above and U.S. Pat. No. 4,460,120. Typical automatic scanning devices require only that they be properly aimed at a bar code target which is to be decoded and that they are manually located the correct distance from the target in order to obtain decoding of the bar code. In typical non-contact scanning systems, a mirror is mounted to the shaft of a drive motor such as a stepper motor operable as a galvanometer. The shaft and attached mirror are oscillated about a predetermined position by a fixed regulated DC current in one phase of the stepper motor and a varying current in another phase. Oscillation of the mirror causes an optical beam directed at the mirror to be deflected across the bar code in an oscillating fashion.
In another type of non-contact scanner, such as that disclosed in U.S. Pat. No. 4,032,888, a resonant electromechanical oscillator causes a mirror mounted to a shaft to oscillate at the resonant frequency of the oscillator. A light beam directed at the mirror is reflected and deflected by the mirror so that it scans across the bar code target at the resonant frequency of the oscillator.
A variable distance automatic scanner is disclosed in U.S. Pat. No. 4,333,006. This patent discloses a multi-focal holographic scanning system which employs a series of varying focal length holograms generated by a rotating disk to focus at differing overlapping distance ranges from the code to be scanned.
Other scanning systems provide for the decoding of bar codes in a variety of vertical planes relative to the scanner. In U.S. Pat. No. 3,902,048, a rotating polygon is provided in which each face of the polygon has a mirrored surface mounted at an angle which varies from face to face of the polygon. As a result of this arrangement, light directed against the rotating polygon is deflected along a variety of predetermined planes, thereby permitting decoding of bar codes which are substantially normal to the plane of the light emitted by the scanner.